This invention relates to double-beam spectrophotometers, and more particularly, to spectrophotometers capable of spectral analysis in the infrared region while eliminating a measurement error caused by undesired radiation.
As is well known in the art, the double-beam spectrophotometers measure the transmittance of a sample by alternately directing a beam of infrared radiation to the sample and a reference or standard material (or an empty cell), detecting the intensity of the sample beam transmitted by the sample and the reference or standard beam transmitted by the reference or standard material, and comparing the sample beam intensity with the reference beam intensity, with the resultant ratio giving the transmittance or absorbance of the sample.
Among such spectrophotometers, thermal infrared detectors such as vacuum thermocouple detectors are often used to carry out analysis in the infrared region. Since both the sample material under analysis and the reference material themselves emit thermal radiation in the infrared region dependent on the ambient temperature and elevated temperatures resulting from illumination, the detector receives the infrared radiation in which the thermal radiation of the sample or reference material itself overlaps the light transmitted by the sample or reference material. Therefore, the output signals of the detector indicative of the intensity of sample and reference beams involve false signals due to thermal radiation. Comparing such detector output signals with each other without any compensation will result in an inaccurate representation of the transmittance of the sample. Particularly when the sample under analysis is at high temperatures, its thermal radiation becomes intense to increase the level of a false signal, thereby introducing a significant error in the measured transmittance of the sample, which is not reliable.
To avoid such inconvenience, so-called emissionless spectrophotometers have been proposed which are adapted to eliminate the influence of undesired radiation.
One typical example is the double-beam spectrophotometer disclosed in Japanese Patent Publication No. SHO 42-23555 (invented by Shigeo Minami, published on Nov. 14, 1967). This spectrophotometer applied a double-chopping system to the well-known optical null balance system to eliminate the influence of unnecessary radiation. In the optical null balance system, generally, a mechanical beam attenuator comprising a wedge-shaped stop or optical wedge driven by a serve motor is inserted in the path of reference beam, the sample beam and the reference beam attenuated by the mechanical attenuator are alternately interrupted by means of a chopper, an alternating current signal is derived out which has an amplitude proportional to the difference between the reference beam intensity and the sample beam intensity, the signal is used as an error signal in a closed loop, and the attenuator is automatically adjusted so that the value of the error signal may become zero. When the attenuator is adjusted in this way, the magnitude of attenuation or the distance of movement of the optical wedge corresponds to the absorbance of the sample, and consequently, changes in the absorbance of the sample can be measured by recording the distance of movement of the attenuator wedge. In such spectrophotometers based on the optical null balance system, according to the invention of the above-mentioned publication, an auxiliary chopper is inserted between the sample and reference cells and the light source in addition to the above-mentioned chopper (main chopper) between the sample and reference cells and the detector. The main chopper switches the beam paths at a given frequency f.sub.1 while the auxiliary chopper is driven so as to switch the beam paths at frequency f.sub.2 lower than frequency f.sub.1. The detector includes a series connection of an amplifier for amplifying a signal having frequency f.sub.1, a synchronous rectifier in synchronism with the main chopper, another amplifier for amplifying a signal having frequency f.sub.2, and another synchronous rectifier in synchronism with the auxiliary chopper in this order. The output of this series circuit, which is used to drive the servo motor of the attenuator, is a signal which is proportional to the difference between the light transmitted by the reference material and the light transmitted by the sample and free of a signal component due to undesired radiation. As a result, there can be obtained a correct absorbance of the sample free of an error induced by undesired radiation.
The above-mentioned spectrophotometers based on the optical null balance system have several drawbacks described below. First of all, since the accuracy of transmittance largely depends on the mechanical accuracy of the attenuator itself as well as the associated drive system, the spectrophotometer is difficult to exhibit highly accurate and stable performance. In connection with this, due to fluctuations in rotation of a driving servo motor for moving a wedge-shaped stop commonly used in the attenuator, error in linearity of a potentiometer for detecting the position of the wedge-shaped stop, and other factors, the distance of movement of the stop is not always proportional to the magnitude of attenuation, often resulting in low accuracy of transmittance measurement. Further, the inclusion of the optical system in the servo loop results in a complicated and expensive apparatus which handles signals in a complicated way and has poor response. In the case of a sample having high absorbance, the sample beam intensity approximates to zero, and accordingly, the reference beam intensity is also attenuated to a level near zero, resulting in reduced loop gain and reduced reliability. In addition, the attenuator itself is reduced in accuracy when the magnitude of attenuation is very high, that is, when the sample has very high absorbance. These undesirably causes a substantial reduction in accuracy of measurement of a high absorbance sample.
Another example of the double-beam spectrophotometers having eliminated the problem of measurement error caused by the undesired radiation of the sample itself is one disclosed in Japanese Patent Publication No. SHO 47-3798 (invented by Michael Allan Ford, published on Feb. 2, 1972). This meter is based on the so-called double-chopping phase control system and includes a split chopper for dividing a beam of light from a light source between sample and reference paths and a recombination chopper for recombining the sample and reference beam paths. The split chopper is in the form of a disc consisting of four quadrants among which one quadrant is transparent, another adjoining quadrant is a reflective region, and the remaining two quadrants are light-shielding regions. The recombination chopper is in the form of a disc divided into two semi-circular transparent and reflective regions. The recombination chopper is rotated at a speed twice the rotation speed of the split chopper. With such choppers cooperated, the detector develops four output signals of S+S.sub.0, R+R.sub.0, S.sub.0, and R.sub.0 in this order with a mutual phase difference of 90 degrees, provided that S represents the light beam transmitted by the sample, S.sub.0 represents the thermal radiation of the sample, R represents the light beam transmitted by the reference material, and R.sub.0 represents the thermal radiation of the reference material. These signals may be isolated and rectified with a phase difference of 90 degrees by means of a synchronous rectifier comprising, for example, a slip ring and a brush, thereby producing S and R signals both free of S.sub.0 and R.sub.0. The transmittance or absorbance of the sample is given as the ratio of these signals. However, the spectrophotometers using the phase control system have the following problems.
When wavelength scanning is generally carried out in spectral analysis, an output waveform of the detector in one cycle sometimes loses its symmetry in a wavelength region where the sample shows high absorbance. In addition, absorption by atmospheric water vapor and carbon dioxide has probably an influence on both the reference and sample beams, distorting the output waveform of the detector in one cycle. In the case of the phase discrimination system, such a distortion of the output waveform largely affects the phase, eventually resulting in a significant error in measurement. This problem will be further explained below. For high absorbance samples, absorbance varies very rapidly during wavelength scanning, resulting in a graded sample beam intensity in one cycle. Since a detector, particularly a thermal infrared detector such as a thermocouple has a large time constant, its output signal appears to have the effect of integrating the intensity waveform of incident light. If the sample beam intensity in one cycle has a gradient as mentioned above, the output waveform is off-centered from the input waveform of the sample beam intensity so that the phase difference between the sample and reference beam intensity components in the output is shifted from 90 degrees, resulting in an error. Also, absorption by atmospheric water vapor and carbon dioxide has an influence on the intensity waveform of incident light to the detector to distort an output signal of the detector to give rise to a similar phase shift, resulting in an error in measurement.
In the case of the phase discrimination system spectro-photometer, if dust deposits on slits in the paths for reference and sample beams and any obstructions in proximity to the beam paths partially intercept beams or mechanical positioning of slits is inaccurate, then the rise or fall of a waveform representative of the intensity of an incident beam to the detector is shifted from the originally set phase, and thus the corresponding output waveform of the detector is deviated, also resulting in an error in measurement. Furthermore, the two choppers mentioned above must be accurately synchronized so as not to leave any phase difference, but in practice, such accurate synchronization is very difficult to achieve without troublesome adjustment.
Accordingly, it is an object of the present invention to provide an emissionless spectrophotometer based on the frequency component detecting system which has eliminated the drawbacks unavoidably associated with prior art emissionless spectrophotometers based on the optical null balance system and the phase control system mentioned above.